Apparatus for liquid development of electrostatic images

ABSTRACT

Apparatus for developing latent electrostatic images formed on the surface of a medium which moves along a development path at a predetermined speed utilizes a plurality of applicators driven at a speed approximating the speed of the medium and arranged so that each applicator is brought into contact with the image bearing surface of the medium and applies thereto a thin layer of liquid developer.

United States Patent [191 York [ APPARATUS FOR LIQUID DEVELOPMENT OF ELECTROSTATIC IMAGES [75] Inventor: William C. York, Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

[22] Filed: Jan. 17, 1974 [21] Appl. No.: 434,163

[52] U.S. C1. 118/637; 117/37 LE; l18/DIG. 23;

355/10 [51] Int. Cl. 6036 15/10 [58] Field of Search 118/637, D10. 23; 117/37 LE; 355/10 [56] References Cited UNITED STATES PATENTS 3,685,907 8/1972 Sato et a1 117/37 LE 3,713,422 1/1973 Sato et al..r...... 118/637 3,722,453 3/1973 Dahlquist et al.... 1l8/DIG. 23 3,750,567 8/1973 Taguchi et al 355/10 (451 July 8,1975

3,750,624 8/1973 Sato et a1 118/637 3,783,827 1/1974 Fukushima et a1. 1. 118/637 3,804,062 4/1974 Fukushima et a1. 1 l8/DIG. 23 3,808,026 4/1974 Sato et a1 355/10 3,815,545 6/1974 Sato et a1 1 18/637 3,816,114 6/1974 Fukushima et a1. 118/637 3,822,670 7/1974 Sato et a1 118/637 Primary Examiner-Mervin Stein Assistant Examiner-Douglas Salser Attorney, Agent, or Firm-Douglas 1 Hague [57] ABSTRACT Apparatus for developing latent electrostatic images formed on the surface of a medium which moves along a development path at a predetermined speed utilizes a plurality of applicators driven at a speed approximating the speed of the medium and arranged so that each applicator is brought into contact with the image bearing surface of the medium and applies thereto a thin layer of liquid developer.

2 Claims, 2 Drawing Figures APPARATUS FOR LIQUID DEVELOPMENT OF ELECTROSTATIC IMAGES BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a developing apparatus and, more particularly, to an improved apparatus for liquid development of latent electrostatic images.

2. Description of the Prior Art Presentation of a thin layer of liquid developer of normal concentration to a latent electrostatic image develops only a faintly visible image when very short development times are used. To increase the optical density of a developed image, it is, therefore, conventional in rapid liquid electrostatic development apparatus to increase the number of toner particles brought to a given incremental charge surface area by providing for relative motion between such toner particles and the latent electrostatic charge pattern. One system to accomplish this result uses a longitudinally serrated gear shaped roller turning in a developer reservoir with a peripheral velocity greater than that of the latent image bearing material which is transported in tangential contact across the top of the roller. The roller acts like a pump to bring many toner particles to the development area, forming a turbulent pool or wave of developer. The turbulence of the developer liquid brings a plurality of toner particles to each discrete charged area of the latent image.

One disadvantage of liquid development techniques using turbulent motion of the liquid developer is their tendency to smear or streak near developed areas. Such smears or streaks in the finished prints are due either to the localized depletion of toner particles in the small space between the development roller and the latent image bearing material or to the collection of toner particles on the print at a site downstream from the developer roller. As the development rate increases to meet the high output rate for electrophotographic machines demanded in todays marketplace, the rate of movement of the latent image bearing material increases proportionately. As expected, the higher speed requirements greatly aggrevate the streaking problem. Another disadvantage of turbulent motion development techniques is their tendency to spuriously deposit toner particles in nonimage background areas resulting in an undesirable spotted or mottled background on the finished print. This effect is particularly pronounced with the use of finely divided toner particles needed to obtain high resolution images.

Another liquid development technique for increasing the density of the developed image wets the latent image bearing surface before development with an insulating clear carrier liquid, the pre-wetting enabling the use of liquid developers of higher toner particle concentration. Highly concentrated liquid developers (above the range of 0.2-5 percent solids) act like paints, showing reduced charge discrimination as concentration increases, and as a result produce copies with an undesirably dense background. Pre-wetting improves the performance of these highly concentrated developers by reducing their tendency toward high background deposition. Pre-wetting does not, however, provide a complete solution to the problem of rapid liquid development of latent electrostatic images.

SUMMARY OF THE INVENTION It is an object of the present invention to provide liquid development apparatus for rapidly developing electrostatic charge patterns.

Another object of the invention is to provide high speed liquid development apparatus which produces high quality prints of improved density and sharpness.

These and other objects are accomplished in accordance with the present invention by providing a plurality of toner laden development applicators each of which applies a thin layer of toner to a flexible latent image bearing medium, for example a web or sheet, in a series of multiple touches". The multiple touches provide high maximum print density without streaking. In a preferred embodiment of the invention, offset guide means, preferably guide rollers, cooperate with development rollers to cause the flexible image bearing medium to slightly wrap around the development rollers during development. Slightly wrapping the charge bearing medium around the development roller surfaces yields better development than a strictly defined tangential contact because there is more time for toner transfer and less demand on the precision of the mating of the guide means and development applicators. The development rollers are moved at a peripheral speed approximating the speed of the image bearing surface to prevent streaking and smearing.

Other objects and advantages of the invention will become more apparent from the detailed description of the preferred embodiments which follow.

BRIEF DESCRIPTION OF THE DRAWINGS In the detailed description of the preferred embodiments presented below, reference is made to the accompanying drawings wherein corresponding parts are identified by like numerals and in which:

FIG. 1 is a cross-section, side elevational view of a liquid development apparatus constructed in accordance with the teachings of the invention; and

FIG. 2 is a schematic illustration of the drive mechanism for the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and particularly to FIG. 1, there is shown a liquid development apparatus D, which includes four 0.790 inch diameter, 35mm length guide or transport rollers 10 and four vertically offset, 0.790 inch diameter, surface roughened, 28mm length steel drill rod development rollers 12. Guide rollers 10 have outwardly protruding tapered flanges 11 at each of their ends which guide a 35mm width electrostatic image bearing medium, for example a web or sheet fed between the rollers 10 and 12. An electrostatic image bearing medium 18, for example a photoconductive web, is fed between the rollers I0 and 12 by two pairs of nip feeding rollers 20, 22 and 24, 26 respectively. Rollers 20 and 24 have solid metal cores 23 coated with a resilient material 25 such as neoprene rubber. Rollers 22 and 26 have flattened flanges 27 at their outer ends and undercut centers, and thus function as rim running rollers. The flanges or rims 27 contact the extreme edges of the medium 18 which is edge stripped with a conducting lacquer to facilitate grounding a conducting layer (not shown) of the medium 18 during charging and development. Rollers l0 and 12 are mounted on shafts l6 protruding from a bearing block 14 formed from an electrically insulating material. The shafts 16 are spaced at one inch distances along the horizontal direction of the bearing block 14 with three-fourths of an inch spacing between the common horizontal planes passing through the axes of the lower development rollers 12 and the upper transport rollers 10. This mounting arrangement will cause a 35mm width image bearing medium 18 fed between the rollers and 12 by nip-feeding rollers 20, 22 and 24, 26 to follow an undulating path through the develop ment apparatus D in which the image bearing surface of the medium 18 contacts each development roller 12 in an area of approximately one-half inch in length. The above described undulating feed path arrangement provides the following development advantages:

First, it provides more precise control of the deposition of several thin layers of toner. The image bearing surface comes repeatedly into wrapped contact with the development electrode roller surfaces, but each roller makes only a single contact with a given charged area, depositing a small amount of toner. Subsequent rollers each add another small deposition of toner with the final obtainable density being a summation of these depositions and therefore dependent on the number of contacts, which in turn is dependent upon the number of development rollers 12. Maximum density depends on the individual toner used and the parameters of the system. In the specific device shown, four contacts give good density images with little added density if over six contacts are used.

Second, the wrapped contact of the medium 18 and development rollers 12 provides more time for development, better development field geometry when electrical bias potentials are applied to the development rollers, and more positive drive of the medium 18 without slippage.

Third, vertically offsetting the transport rollers 10 from the development rollers 12 enables the development of either short lengths of flexible material or continuous 35mm webs without getting the rear surface of the photoconductive material soiled with toner. Having the length of the development rollers 12 (28mm) shorter than the width of the medium 18 (35mm) also helps to keep toner from the rear surface of the web.

Fourth, roughening the outer surface of development rollers 12 as by sandblasting or fine knurling promotes wetting" by the liquid developer 42 which enables a more uniform coating of developer on the surfaces of the rollers.

As also shown in FIG. 1, development apparatus D further includes a nylon insulating, surface roughened roller 28 and two free wheeling, ball-bearing rollers 30 and 32 which align the medium 18 prior to contact with the roller 28 and prior to contact with the nip feeding rollers 24 and 26, respectively.

Referring now to FIG. 2, the drive system of development apparatus D is mounted on the rear surface of bearing block 14 and includes two electrically insulating timing belts 46 and 48 which drive roller sets 20, 10 and 24, and 22, 12 and 26, respectively. Timing belts 46 and 48 are in turn driven by a drive train 50 comprising a pulley 51 mounted on the shaft 55 of nip feeding roller 22 and driven by a belt 56 running between the pulley 51 and a drive motor (not shown) and a pair of meshed gears 52 and 54 mounted respectively on shaft 55 and on the shaft 57 of nip feeding roller 20.

The afore-described drive system drives rubber covered nip feeding rollers 20 and 24, rim-running nip feeding rollers 22 and 26, transport rollers 10, development rollers 12 and wetting roller 28 at the same velocity. The velocity of the driven development rollers 12 is, therefore, equal to the medium transport velocity. Driving the medium 18 and development rollers 12 at the same velocity eliminates smearing of the toned image by eliminating relative motion between the rollers and the medium.

Two trays 34 and 36, respectively, are slidably mounted on bearing block 14 by conventional means (not shown). Tray 34 together with wetting roller 28 and a coating roller 44 form an optional predevelopment station 38 which prior to development, wets the latent image bearing surface with an insulating clear carrier liquid. As is known in the art, see for example British Pat. No. 1,118,812, such predevelopment treatment enables the use of developers with an increased concentration of solid toner particles. The predevelopment station 38 is described in detail at a later portion of the specification. Tray 36 contains the liquid developer 42. Free-wheeling k inch diameter, metal coating rollers 40 are partially immersed in the liquid developer 42 contained in the tray 36. Rollers 40 are pushed up against the development rollers 12 by spring tension (not shown) so that the rollers 12 are coated with a thin layer of developer as they drive rollers 40. It is to be noted, however, that the coating rollers 40 and 44 may be eliminated if desired, in which case, the trays 34 and 36 partially immerse the rollers 28 and 12, respectively, in their contents. if the coating rollers are eliminated, it is desirable to introduce 60-80 mesh spherical iron powder into the tray 36 at the higher toner particle concentrations. The iron powder prevents the gelling of the toner particles in the developer mixture and helps to apply a uniform coat of developer to the rollers 12.

To assist the coating of positively charged toner onto the development rollers 12 (assuming a negatively charged latent image is to be developed) and to prevent the deposition of toner on undischarged background areas of the photoconductive strip 18, a negative bias potential may be applied to the rollers 12 and a positive bias potential applied to the tray 36 by a source of potential 58 while the transport rollers 10 and hence the photoconductors conducting interlayer are held at ground potential. Bias voltages of requisite magnitude and potential may also be applied to the rollers 12 and tray 36 from the source of potential 58 to produce reversal prints.

Use of the optional pre-wet treatment station 38 permits the concentration of toner particles in the liquid developer 42 to be increased to 0.2-10 percent solids in some cases without toner deposition at the image free background areas of the medium 18. The nylon roller 28, contacts the medium 18 without altering the charge pattern carried thereon. Roller 28 is wet by an offset coating roller 44 partially immersed in the carrier liquid, e.g., lsopar G, contained in the tray 34. (lsopar G is a trademark of Humble Oil and Refining Company used to designate a synthetic hydrocarbon liquid of high purity characterized by a Kauri-Butanol value of about 27, a flash point of about 104F, an evaporation rate of percent concentration of about 3,180 seconds, a boiling range of from 318 to 350F, and a low toxicity).

The operation of the invention is illustrated by the following example. The tray 36 was filled with a 3 percent total solids content liquid developer comprising a dilution of a pigmented mixtureiof an alkyd resin with a phenoI-fomtaldehyde resin dispersed in an lsopar G carrier liquid at a concentration of 3.0 grams of total solids per liter. Tray 34 was filled with clear lsopar G. Bias voltages of 50 volts and +100 volts were applied to the rollers 12 and tray 36, respectively. A 35mm strip of an organic photoconductive element was prepared in accordance with the teachings of US. Pat. NO. 3,542,547, issued Nov. 24, 1970 in the name of Charles V. Wilson, incorporated herein by reference. The organic photoconductor was dispersed in Vitel 101, an insulating polyester resin obtained from Goodyear Tire and Rubber Co., and coated unsensitized on a polyester support. The photoconductor was charged to a surface potential of -700 volts and contact ex posed to a positive-appearing line image. The photoconductive web was then developed at a linear rate of 60 feet/minute. Sharp images of good density and low background were obtained.

From the foregoing, the beneficial effects of the present invention are readily apparent. Novel development apparatus has been disclosed which rapidly develops latent electrostatic images with no surface smear. The multiple touch-down" development technique provides precise control of the deposition of several thin, uniformly consistent layers of toner. The resulting prints are, therefore, of improved density and sharpness. The apparatus is self-threading for any length of flexible sheet or web that exceeds the spacing of the rollers. The apparatus does not require the use of a pump to move liquid developer into contact with the photoconductive element and is, therefore, of simple and economical design.

The invention has been described in detail with reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, other web widths could be developed with longer rollers. If desired, the development apparatus D can also include a post-development treatment station to overcoat or lacquer liquid systems to afiord protection to the finished print or to make stripable layers.

I claim:

1. A development apparatus for developing latent electrostatic images borne on an image-bearing surface of a flexible medium during movement along a development path, said apparatus comprising:

a. a plurality of rotatable development rollers, lo-

cated in horizontally spaced relation generally below the development path and having a diameter such that upper peripheral portions thereof define a generally horizontal lower side of the development path, each of said development rollers having a length which extends across the image bearing area but is shorter than the width of the flexible medium;

b. a plurality of guide rollers located in horizontally spaced relation generally above the development path and having a diameter such that lower peripheral portions thereof define a generally horizontal upper side of the development path, the axis of rotation of said quide rollers and said development rollers being offset along the direction of feed and spaced vertically so as define a horizontally, undulating development path which causes a flexible medium fed along said path to partially wrap around each development roller;

c. means for applying to the outer periphery of each development roller a thin layer of liquid developer of the type comprising toner particles dispersed in an insulating liquid carrier; and

d. drive means for transporting the flexible medium along said development path at a predetermined rate with the image-bearing surface facing said development rollers and for rotating said development rollers at a peripheral speed substantially equal to said predetermined rate;

whereby said development rollers can apply a plurality of layers of liquid developer to the imagebearing surface without wetting the opposite side of the flexible medium with the liquid developer.

2. Apparatus as defined in claim 1, wherein said guide rollers are offset from said development rollers, in the direction of feed, a distance less than the length of the flexible medium.

a it is s 

1. A development apparatus for developing latent electrostatic images borne on an image-bearing surface of a flexible medium during movement along a development path, said apparatus comprising: a. a plurality of rotatable development rollers, located in horizontally spaced relation generally below the development path and having a diameter such that upper peripheral portions thereof define a generally horizontal lower side of the development path, each of said development rollers having a length which extends across the image bearing area but is shorter than the width of the flexible medium; b. a plurality of guide rollers located in horizontally spaced relation generally above the development path and having a diameter such that lower peripheral portions thereof define a generally horizontal upper side of the development path, the axis of rotation of said quide rollers and said development rollers being offset along the direction of feed and spaced vertically so as define a horizontally, undulating development path which causes a flexible medium fed along said path to partially wrap around each development roller; c. means for applying to the outer periphery of each development roller a thin layer of liquid developer of the type comprising toner particles dispersed in an insulating liquid carrier; and d. drive means for transporting the flexible medium along said development path at a predetermined rate with the image-bearing surface facing said development rollers and for rotating said development rollers at a peripheral speed substantially equal to said predetermined rate; whereby said development rollers can apply a plurality of layers of liquid developer to the image-bearing surface without wetting the opposite side of the flexible medium with the liquid developer.
 2. Apparatus as defined in claim 1, wherein said guide rollers are offset from said development rollers, in the direction of feed, a distance less than the length of the flexible medium. 